The oxygen reduction reaction (ORR) has been studied on single crystals of the ternary alloys Pt45Pd45Co10 and Pt77Pd8Co15 in O2-saturated 0.1 M HClO4 using a hanging meniscus rotating disk electrode. Specific activity for the ORR depends markedly on the composition and surface structure of the single crystal electrodes. The activity on Pt45Pd45Co10(hkl) increases in the order Pt45Pd45Co10(1 0 0) ≈ Pt45Pd45Co10(1 1 1) < Pt45Pd45Co10(1 1 0), whereas the activity on Pt77Pd8Co15(hkl) is Pt77Pd8Co15(1 0 0) < Pt77Pd8Co15(1 1 0) < Pt77Pd8Co15(1 1 1). The ORR activity of Pt45Pd45Co10(1 1 0) is 2.6 times higher than that of Pt3Co(1 1 0), which is known to have high ORR activity. The activity of Pt77Pd8Co15(1 1 1) is 2.4 times higher than that of Pt3Co(1 1 1). Low energy ion scattering spectroscopy (LE-ISS) shows that Pt and Co coexist at the topmost layer of Pt77Pd8Co15(1 1 1). Pt, Pd and Co atoms are dissolved uniformly in the inner layers according to elemental mapping by scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS).
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